In the mobile communications field including PHS and mobile phones, piezoelectric oxide single crystal wafers of lithium tantalate, lithium niobate, quartz, lithium tetraborate, langasite or the like are utilized as substrates of surface acoustic wave devices.
The SAW device includes a piezoelectric oxide single crystal wafer as a substrate and interdigital electrodes deposited on one major surface of the substrate to provide a transducer. Since the device is constructed such that the transducer receives and excites surface acoustic waves, the transducer-forming surface of the single crystal wafer must be mirror polished. If the back surface of the single crystal wafer is similarly mirror polished, the transducer receives and excites unnecessary waves (or interference waves) such as bulk waves at the same time as it receives and excites surface acoustic waves, incurring spurious disturbance in frequency response. Then, single crystal wafers for SAW devices are roughened on their back surface by lapping or honing with coarse abrasive grains.
Specifically, these wafers are traditionally fabricated as follows. A single crystal ingot is first prepared by art-recognized methods such as Czochralski method. Using an inner diameter slicing or wire saw, the ingot is cut into a disc-shaped wafer, which is lapped on both surfaces to a predetermined thickness. Then only one surface of the wafer is mirror polished by an abrasive tool, followed by cleaning.
In unison with the recent trend that equipment such as PHS and mobile phones are reduced in size and profile, SAW devices built therein are also reduced in profile. There thus exists an urgent requirement to reduce the thickness of piezoelectric oxide single crystal wafers for use in SAW filters or the like.
In the case of lithium tantalate single crystal wafers for use in SAW filters or the like, for example, wafers of 350 microns thick are common at the present while efforts have been initiated to produce thinner wafers with a thickness of around 250 microns.
In the SAW-utilizing devices described above, the necessary thickness of piezoelectric oxide single crystal is computed to be 10 times the wavelength on theory. This, combined with the state-of-the-art, suggests that a thickness of around 50 microns is sufficient. While piezoelectric oxide single crystal wafers having a thickness of 350 microns are supplied, some device manufacturers have started implementing a process of forming a metal electrode pattern on the mirror finish surface of such a thick wafer by photolithography or the like, and finally removing an unnecessary portion from the wafer back side by a surface grinding technique or the like.
Under such circumstances, if a wafer manufacturer supplies thinner piezoelectric oxide single crystal wafers, they are expected to contribute to a reduction of process steps on the device manufacturer side. However, as piezoelectric oxide single crystal wafers become thinner, they become lower in mechanical strength and more susceptible to failure during the wafer manufacturing process, so that the manufacturing yield can be reduced.
In particular, the process of working piezoelectric oxide single crystal wafers for use in SAW devices has the following problem. While a relatively thick disc-shaped wafer is furnished by cutting, it must be worked to the target thickness by post-steps of lapping and mirror polishing. An increased risk of wafer failure resulting from reduced mechanical strength is of concern.
As discussed above, when piezoelectric oxide single crystal wafers are used as SAW filters, the transducer-forming surface of the wafer must be mirror polished because the transducer receives and excites surface acoustic waves. On the other hand, the back surface of the wafer must be roughened by lapping or honing with coarse abrasive grains in order to exclude unnecessary waves (or interference waves) such as bulk waves which are received and excited at the same time as are surface acoustic waves.
Prior Art 1: JU-A 58-129658
Prior Art 2: JP-A 2-98926
Prior Art 3: JP-A 62-297064
Prior Art 4: JP-A 63-93562